Doc: Multi-Dim and Python Executable (#152)

Documents how to overwrite the SPACEDIM default of AMReX from "3"
to building 1, 2 and 3 dimensional
versions at the same time.
Example: `-DAMReX_SPACEDIM="1;2;3"`

Documents how to overwrite the automatically found Python executable,
which is useful if one is picked that is undesired. Typically happens
outside of virtual environments and on macOS.
Example: `-DPython_EXECUTABLE=$(which python3)`

README.md

@@ -135,7 +135,7 @@ python3 -m pip install -v --force-reinstall --no-deps .
 
 If you are iterating on builds, it will faster to rely on ``ccache`` and to let CMake call the ``pip`` install logic:
 ```bash
-cmake -S . -B build
+cmake -S . -B build -DAMReX_SPACEDIM="1;2;3"
 cmake --build build --target pip_install -j 8
 ```
 
@@ -168,7 +168,7 @@ If you are using the pip-driven install, selected [AMReX CMake options](https://
 | `AMREX_GPU_BACKEND` | **NONE**/SYCL/CUDA/HIP | On-node, accelerated GPU backend |
 | `AMREX_MPI` | ON/**OFF** | Enable MPI |
 | `AMREX_PRECISION` | SINGLE/**DOUBLE** | Precision of AMReX Real type |
-| `AMREX_SPACEDIM` | 1/2/**3** | Dimension of AMReX   |
+| `AMREX_SPACEDIM` | "1;2;3"  | Dimension(s) of AMReX as a ``;``-separated list |
 | `AMREX_BUILD_SHARED_LIBS` | ON/**OFF** | Build the core AMReX library as shared library |
 | `AMREX_SRC` | *None* | Absolute path to AMReX source directory (preferred if set) |
 | `AMREX_REPO` | `https://github.com/AMReX-Codes/amrex.git` | Repository URI to pull and build AMReX from |
@@ -184,6 +184,7 @@ Furthermore, pyAMReX adds a few selected CMake build options:
 
 | CMake Option | Default & Values | Description |
 |------------------------------|--------------------------------------------|---------------------------------------------------------------|
+| `AMReX_SPACEDIM` | **3**, use `"1;2;3"` for all | Dimension(s) of AMReX as a ``;``-separated list |
 | `pyAMReX_IPO` | **ON**/OFF | Compile with interprocedural/link optimization (IPO/LTO) |
 | `pyAMReX_amrex_src` | *None* | Absolute path to AMReX source directory (preferred if set) |
 | `pyAMReX_amrex_internal` | **ON**/OFF | Needs a pre-installed AMReX library if set to `OFF` |
@@ -193,6 +194,7 @@ Furthermore, pyAMReX adds a few selected CMake build options:
 | `pyAMReX_pybind11_internal` | **ON**/OFF | Needs a pre-installed pybind11 library if set to `OFF` |
 | `pyAMReX_pybind11_repo` | `https://github.com/pybind/pybind11.git` | Repository URI to pull and build pybind11 from |
 | `pyAMReX_pybind11_branch` | `v2.10.1` | Repository branch for `pyAMReX_pybind11_repo` |
+| `Python_EXECUTABLE` | (newest found) | Path to Python executable |
 
 As one example, one can also build against a local AMReX copy.
 Assuming AMReX' source is located in `$HOME/src/amrex`, then `export AMREX_SRC=$HOME/src/amrex`.